The present invention pertains to failure protection circuitry for microprocessor-based equipment, such as encryption equipment and the like. Prior encryption units make extensive use of complex, discrete logic and circuitry to provide essential failure protection against compromising emanations from the unit when one or more device failures occur in the unit while in service. This circuitry consists of numerous checking features on critical operations along with checking logic to check the prime checker.
There is a strong desire to use off-the-shelf (low cost) microprocessors to replace the large amount of discrete logic devices or special, custom LSI devices. However, at the present, a substantial problem exists in that off-the-shelf microprocessor LSI devices, when analyzed using fail safe procedures, cannot be modified by adding circuitry to internal paths in order to prevent security compromises because of a device failure.